Sulfur dioxide absorbent is an ionic liquid including a salt of a diamine compound that is substituted with a hydroxyl group

ABSTRACT

A sulfur dioxide absorbent that is an ionic liquid including a solvent; and a salt of a diamine compound that is substituted with a hydroxyl group and has a chemical formula 1 to 3 below dissolved in the solvent: 
     
       
         
         
             
             
         
       
         
         
           
             where, in Chemical Formula 1 and 2, R 1 -R 4  are the same or different and each is independently selected from the group consisting of H, a C1-C6 alkyl, and a C1-C6 alkoxy; and where, in Chemical Formula 1 to 3, X is selected from the group consisting of Cl, Br, I, MeSO 3 , CF 3 SO 3 , HCO 2 , CF 3 CO 2  and CH 3 CO 2 ; and n is an integer of 1-10. The sulfur dioxide absorbent is constituted to selectively absorb sulfur dioxide and sulfurous acid (H 2 SO 3 ) formed by combination of sulfur dioxide with water, not CO 2 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to Korean PatentApplication No. 10-2018-0143845 filed on Nov. 20, 2018 in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to an absorbent capable of removing notonly sulfur dioxide (SO₂) contained in combustion exhaust gas or in theair but also sulfurous acid (H₂SO₃) formed by combination of sulfurdioxide with water. Particularly, the absorbent uses an ionic liquid inthe form of a salt including a diamine compound substituted with ahydroxyl group.

BACKGROUND

In fossil fuel, such as coal and petroleum, a significant amount ofsulfur compounds is contained and such sulfur compounds are emitted inthe form of sulfur dioxide (SO₂) which causes acid rain duringproduction of energy through burning of fuel. Therefore, in most powerplants or industrial facilities where fossil fuel is burned,desulfurized exhaust gas is emitted into the air to prevent emission ofa main cause of air pollution.

Use of lime (CaO) or limestone (CaCO₃) is known as a method fordesulfurization of exhaust gas from a power plant. Use of such calciumcompounds is advisable in terms of cost efficiency. However, since suchcalcium compounds have a significantly low solubility to water of about0.17% (CaO) and about 0.0013% (CaCO₃), they are pulverized into verysmall particles and then formed into slurry in water so that they mayreact with SO₂. Herein, reaction with SO₂ is a solid-liquid reaction,and thus does not show a high reaction rate as compared to aliquid-liquid reaction. In addition, use of an excessive amount of watercauses a problem related with waste water and such low solubility towater causes a serious problem of pipe blockage during the overallprocess.

As a liquid SO₂ absorbent, an amine-based compound has been used mostwidely. Particular examples of such absorbents include triethanolaminedisclosed in U.S. Pat. No. 3,904,735, monoethanolamine disclosed in U.S.Pat. No. 4,201,752, and diethanolamine disclosed in U.S. Pat. No.2,404,854. The reason why such an amine absorbent is used as SO₂absorbent is that amine bound chemically with SO₂ undergoes a cleavageof binding under heating, and thus SO₂ may be desorbed and collected andthe absorbent may be regenerated. However, the amine process has thefollowing disadvantages: a high absorbent regeneration temperaturecaused by strong binding between amine and sulfur dioxide andirreversible decomposition of amine; degradation of performance of theabsorbent and supplement of the absorbent; corrosion of an absorptionsystem caused by the amine itself or decomposition products thereof; andcontamination of regenerated gas with the vapor pressure of the amineabsorbent.

Recently, in order to overcome such disadvantages of the conventionalabsorbent, there has been an attempt to use an ionic liquid, which showsno volatility, has high thermal and chemical stability and maintains aliquid phase at a low temperature of 100° C., as an absorbent, asdisclosed in U.S. Pat. Nos. 6,849,774, 6,623,659, and US PatentPublication No. 2008-0146849.

In addition, according to Angew. Chem., Int. Ed. (2004, 43, 2415-2417),use of 1,1,3,3-tetramethylguanidinium lactate ([TMG]L) provides an SO₂absorption ratio of 0.978 mol per mole of ionic liquid. According toKorean Patent No. 10-0831093, [BMIm]Cl provides an SO₂ absorption ratioof 1.68 mol per mole of ionic liquid and [EMIm]EtSO₄ provides arelatively low SO₂ absorption ratio of 0.92 mol per mole of ionicliquid. Further, according to Korean Patent Publication No.10-2010-0043796, it is reported that a fluorine-containing ionic liquid,[BhFplm]CF₃CO₂ (1-butyl-3-hexafluoropropyl imidazolium trifluoroacetate)has high stability against heat and sulfur dioxide and absorbs 0.48 molof sulfur dioxide per mol of ionic liquid.

However, since an ionic liquid having a fluorine-containing anion, suchas tetrafluoroborate (BF₄), is decomposed easily by moisture to producehydrofluoric acid (HF), the ionic liquid as an absorbent may be lost andhas a difficulty in regeneration. When the anion is Cl⁻, the ionicliquid is present in the form of solid at room temperature. Thus, thereis a problem in that it is required to maintain the absorptiontemperature to 50° C. or higher.

Energy Environ. Sci., 4, 1802-1806 (2011) and Chem. Commun., 2633-2635(2012) discloses a method for increasing absorption ratio of SO₂ byusing an ionic liquid having an ether functional group or ionic liquidhaving a tetrazole ion. It is reported that each ionic liquid can absorbSO₂ up to 4 mol per mol at 1 bar.

The ionic liquids having such high SO₂ absorption ratio arecharacterized in that a Lewis base site capable of interacting with SO₂having Lewis acid property is present in the molecule. Therefore,molecules having a large number of Lewis base sites per unit moleculeshow high SO₂ absorption ratio.

Meanwhile, exhaust gas includes 5-15% of moisture in addition to SO₂.The presence of moisture may be concentrated together with an SO₂absorbent during the treatment of SO₂, wherein SO₂ reacts with water sothat it is converted into a strong acid, H₂SO₃. H₂SO₃ has quitedifferent chemical properties from SO₂. In other words, H₂SO₃ does notfunction as a Lewis acid, and thus it cannot be treated with theconventional SO₂ absorbent. On the contrary, when aqueous H₂SO₃ solutionis treated with a primary to tertiary amine or base, it is possible toisolate H₂SO₃ through acid-base reaction. However, in this case, theproduced salt is present in a form dissolved in water. As a result, itis required to further carry out separation of the salt from water.

Most of the conventional SO₂ absorbents are miscible with water or aredecomposed in the presence of water. In addition, absorption ratio ofsuch absorbents for SO₂ present in the form of an acid, such as H₂SO₃,is not known. In fact, Chem Commun 2012, 48, 1227 discloses a method fortreating SO₂ by using a diethylene triamine/P66614CI system. Herein, itis stated that when water is present in the ionic liquid or the otheramine solvent, acid-base reaction occurs between amine and H₂SO₃, notSO₂.

REFERENCES

Korean Patent No. 10-0831093

Korean Patent Laid-Open No. 10-2010-0043796

SUMMARY

An embodiment of the present disclosure is directed to providing anabsorbent capable of removing not only sulfur dioxide (SO₂) contained incombustion exhaust gas or in the air but also sulfurous acid (H₂SO₃)formed by combination of sulfur dioxide with water.

Another embodiment of the present disclosure is directed to providing amethod for preparing the absorbent.

In one aspect, there is provided a sulfur dioxide absorbent selectedfrom the group consisting of the following [Chemical Formula 1] to[Chemical Formula 3]:

In Chemical Formula 1 to Chemical Formula 3, R₁-R₄ are the same ordifferent and each is independently selected from the group consistingof H, a C1-C6 alkyl, carbonyl and a C1-C6 alkoxy; X is selected from thegroup consisting of Cl, Br, I, MeSO₃, CF₃SO₃, CF₃CO₂, HCO₂, CF₃CO₂ andCH₃CO₂; and n is an integer of 1-10.

The sulfur dioxide absorbent may be one that absorbs sulfur dioxide andsulfurous acid (H₂SO₃) formed by combination of sulfur dioxide withwater.

The sulfur dioxide absorption temperature may be 10-80° C. and thesulfur dioxide desorption temperature may be 50-150° C.

In another aspect, there is provided a method for preparing the sulfurdioxide absorbent, including a step of mixing a compound represented bythe following [Chemical Formula 12] with a compound represented by thefollowing [Chemical Formula 15] at a molar ratio of 1:1-5 to form asalt:

In Chemical Formula 12 and Chemical Formula 15, R₁-R₄ are the same ordifferent and each is independently selected from the group consistingof H, a C1-C6 alkyl, carbonyl and a C1-C6 alkoxy; Y₁ is selected fromthe group consisting of Cl, Br and I; and n is an integer of 1-10.

In a variant, the method may include a step of mixing a compoundrepresented by the following [Chemical Formula 13] with a compoundrepresented by the following [Chemical Formula 15] at a molar ratio of1:1-5 to form a salt:

In Chemical Formula 13 and Chemical Formula 15, R₁-R₂ are the same ordifferent and each is independently selected from the group consistingof H, a C1-C6 alkyl, carbonyl and a C1-C6 alkoxy; Y₁ is selected fromthe group consisting of Cl, Br and I; and n is an integer of 1-10.

In another variant, the method may include a step of mixing a compoundrepresented by the following [Chemical Formula 14] with a compoundrepresented by the following [Chemical Formula 15] at a molar ratio of1:1-5 to form a salt:

In Chemical Formula 14 and Chemical Formula 15, Y₁ is selected from thegroup consisting of Cl, Br and I; and n is an integer of 1-10.

According to an embodiment, the salt compound obtained after thecompletion of the reaction may be reacted with a compound represented bythe following [Chemical Formula 16] to exchange the anion of the saltcompound:MY₂  [Chemical Formula 16]

In Chemical Formula 16, M is selected from the group consisting of Li, Kand Na; and Y₂ is selected from the group consisting of MeSO₃, CF₃SO₃,CF₃CO₂, HCO₂, CF₃CO₂ and CH₃CO₂.

According to the present disclosure, a molten salt of a diamine compoundsubstituted with a hydroxyl group is dissolved in a solvent to form anionic liquid (absorbent). The ionic liquid is used to remove not onlysulfur dioxide (SO₂) contained in combustion exhaust gas or in the airbut also sulfurous acid (H₂SO₃) formed by combination of sulfur dioxidewith water.

In addition, the sulfur dioxide and sulfurous acid absorbent accordingto the present disclosure is advantageous in that it shows excellentabsorption ratio even at a low SO₂ concentration, allows easy desorptionof the absorbed SO₂, and maintains its absorption ratio to a levelsimilar to the initial absorption ratio without loss even after beingused repeatedly. Further, even when the absorbent according to thepresent disclosure is dissolved in water at a high concentration, it isnot solidified, and thus can prevent a pipe blockage phenomenon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows NMR data determined for the molten salt prepared accordingto Example 1.

FIG. 2 shows NMR data of the ionic liquid prepared from the molten saltaccording to Example 1 after the reaction with SO₂.

FIG. 3 is a graph illustrating the results of repeating SO₂ absorptionand desorption 4 times by using the ionic liquid prepared from themolten salt according to Example 1.

FIG. 4 shows NMR data illustrating the results of repeating SO₂absorption and desorption 5 times by using the ionic liquid preparedfrom the molten salt according to Example 1.

FIG. 5 is a graph illustrating the SO₂ concentration obtained when theionic liquid prepared from the molten salt according to Example 1 orwater is applied to a SO₂ absorption-desorption system in the presenceof gas containing SO₂ at a high concentration.

FIG. 6A shows a photographic image taken after the reaction of the ionicliquid containing a different concentration of hydroxyl group-freediamine salt with SO₂, and FIG. 6B shows a photographic image takenafter the reaction of the ionic liquid containing a differentconcentration of the molten salt according to Example 1 with SO₂.

FIG. 7 shows an absorption reactor.

DETAILED DESCRIPTION OF EMBODIMENTS

The present disclosure relates to an absorbent capable of removing notonly sulfur dioxide (SO₂) contained in combustion exhaust gas or in theair but also sulfurous acid (H₂SO₃) formed by combination of sulfurdioxide with water.

The solid-based absorbent, such as CaO or CaCO₃, widely used currentlyin the industrial field has low solubility to water and shows a lowabsorption rate, and is disadvantageous in that handling of solid isrequired. In the case of an amine-based absorbent, it has highabsorption ratio to sulfur dioxide but it is present in the form of asalt. Thus, there are problem in that the absorbent may be dischargedtogether upon the desorption of SO₂ to cause a loss of absorbent, thedesorbed gas may be contaminated with the absorbent to cause degradationof purity, and amine may be decomposed irreversibly due to the strongbinding between amine and SO₂.

As a method for overcoming the above-mentioned disadvantages of theamine-based absorbent, there have been continuous attempts to use aroom-temperature ionic liquid as a sulfur dioxide absorbent. Thesolubility of the gas absorbed by the ionic liquid is varied dependingon a degree of interaction between the gas and ionic liquid. Thus, it ispossible to control the solubility to a specific gas by modifying thecation and anion of the ionic liquid so that the polarity, acidity,basicity and nucleophilicity of the ionic liquid may be adjusted.However, such ionic liquids have some problems, since they aresusceptible to moisture and decomposed with ease, they show a low SO₂absorption ratio, and they are too expensive to be used in generalapplications. In addition, some ionic liquids require a high desorptiontemperature due to excessively strong binding with SO₂, and thus has aproblem of a difficulty in regeneration thereof.

According to the present disclosure, there is provided an amine saltcompound with an ionic structure having high cost-efficiency, highdurability and excellent SO₂ absorption ratio in order to overcome theabove-mentioned problems related with the conventional ionic liquids.After studying the problems of the conventional amine-based ionicliquids and the sulfur dioxide absorption mechanism intensively, it hasbeen found that use of a molten salt including a hydroxylgroup-containing diamine-based cation and an anion, more particularly amolten salt having a compound structure selected from the following[Chemical Formula 1] to [Chemical Formula 3], allows not only reversiblecollection of a significantly low concentration of SO₂ but alsoselective removal of SO₂ even in the presence of a large amount of CO₂.

Hereinafter, the present disclosure will be explained in detail.

The sulfur dioxide and sulfurous acid absorbent according to the presentdisclosure is selected from the group consisting of the following[Chemical Formula 1] to [Chemical Formula 3]:

In Chemical Formula 1 to Chemical Formula 3, R₁-R₄ are the same ordifferent and each is independently selected from the group consistingof H, a C1-C6 alkyl, carbonyl and a C1-C6 alkoxy; X is selected from thegroup consisting of Cl, Br, I, MeSO₃, CF₃SO₃, CF₃CO₂, HCO₂, CF₃CO₂ andCH₃CO₂; and n is an integer of 1-10.

Particularly, the compound selected from the group consisting of thefollowing [Chemical Formula 1] to [Chemical Formula 3] may be selectedfrom the group consisting of the following [Chemical Formula 4] to[Chemical Formula 11]:

In addition, the sulfur dioxide and sulfurous acid absorbent accordingto the present disclosure is obtained through a step of forming a saltof a diamine compound substituted with a hydroxyl group. According to anembodiment, the absorbent is obtained by a method including a step ofmixing a compound represented by the following [Chemical Formula 12]with a compound represented by the following [Chemical Formula 15] at amolar ratio of 1:1-5, preferably 1:1-3, to form a salt.

According to another embodiment, the absorbent is obtained by a methodincluding a step of mixing a compound represented by the following[Chemical Formula 13] with a compound represented by the following[Chemical Formula 15] at a molar ratio of 1:1-5, preferably 1:1-3, toform a salt. According to still another embodiment, the absorbent isobtained by a method including a step of mixing a compound representedby the following [Chemical Formula 14] with a compound represented bythe following [Chemical Formula 15] at a molar ratio of 1:1-5,preferably 1:1-3, to form a salt.

In Chemical Formula 12 to Chemical Formula 15, R₁-R₄ are the same ordifferent and each is independently selected from the group consistingof H, a C1-C6 alkyl, carbonyl and a C1-C6 alkoxy; Y₁ is selected fromthe group consisting of Cl, Br and I; and n is an integer of 1-10.

When the molar ratio is not within the above-defined range in formingthe salt compound, a large amount of products other than the desiredsalt compound may be produced and side reactions may occur frequently tocause a decrease in yield.

It is possible to obtain a compound selected from the group consistingof the above [Chemical Formula 1] to [Chemical Formula 3] by optionallyexchanging the anion of the salt compound after the completion of thereaction. For example, the anion of the salt compound after thecompletion of the reaction is a halogen anion. When the halogen anion isexchanged with another anion, the salt compound is allowed to react witha compound represented by the following [Chemical Formula 16]:MY₂  [Chemical Formula 16]

In Chemical Formula 16, M is selected from the group consisting of Li, Kand Na; and Y₂ is selected from the group consisting of MeSO₃, CF₃SO₃,CF₃CO₂, HCO₂, CF₃CO₂ and CH₃CO₂.

The absorbent according to the present disclosure is an ionic liquid,and can absorb SO₂ in the absence of water when removing SO₂ fromexhaust gas. However, the absorbent according to the present disclosurecan remove SO₂ in the presence of water. When removing SO₂ in theabsence of water, the tertiary amine of the absorbent can absorb SO₂through the physical interaction with SO₂. In addition, in the presenceof water, SO₂ reacts with water first and is present as H₂SO₃. Then,H₂SO₃, which is a strong acid, reacts with the tertiary amine group ofthe absorbent so that it may be converted into the structure of[R₃NH⁺][HSO₃ ⁻]. Herein, when using the ionic liquid (absorbent)according to the present disclosure, the absorbent is isolated after thereaction, and then is heated to 100° C. or more. Thus, it is possible toregenerate the absorbed liquid at a relatively low temperature.

Desorption of SO₂ absorbed to the ionic liquid according to the presentdisclosure requires significantly lower energy as compared to thehigh-temperature stripping step required for recovery of SO₂ in aprocess using an amine absorbent. This is because removal of SO₂ fromthe ionic liquid to which SO₂ is absorbed through physical interactionis significantly easier than removal of SO₂ from a primary or secondaryamine solution to which SO₂ is chemically bound. Particularly, atertiary amine as a substituent of a quaternary amine group has asignificantly lower basicity as compared to a general tertiary amine.Therefore, it shows lower interaction intensity with SO₂, which resultsin a relatively lower regeneration temperature.

In addition, when removing H₂SO₃, a structure of SO₂ dissolved in water,the resultant [R₃NH⁺][HSO₃ ⁻] salt is a weak base-strong acidcombination, and thus allows regeneration at a relatively lowtemperature.

When absorbing SO₂ by using the absorbent according to the presentdisclosure, the absorption temperature is 10-80° C., preferably 30-50°C. When the absorption temperature is lower than 10° C., additionalenergy may be consumed to reduce the temperature of exhaust gas. Whenthe absorption temperature is higher than 80° C., absorption anddesorption may be performed at the same time to cause a decrease in SO₂absorption ratio. The absorption pressure preferably ranges from ambientpressure to 3 atm, considering the vapor pressure (3.44 atm at 21° C.)of SO₂.

Further, SO₂ desorption temperature is 50-150° C., preferably 70-120° C.When desorption temperature is lower than 50° C., desorption ratio maybe decreased. When desorption temperature is higher than 150° C., higherenergy is required. Thus, it is preferred to maintain the above-definedtemperature range. When absorbing SO₂, SO₂ absorption is increased asthe temperature is decreased and the pressure is increased.Particularly, when absorption pressure is increased, SO₂ absorption isincreased in almost linearly proportion to the pressure. However, sinceSO₂ partial pressure is not high in most of the processes requiringremoval of SO₂, it is preferred to use an ionic liquid having a high SO₂absorption ratio even at low pressure.

The examples and experiments will now be described. The followingexamples and experiments are for illustrative purposes only and notintended to limit the scope of this disclosure. In addition, it will beapparent to those skilled in the art that various changes andmodifications may be made based on the disclosure of the presentinvention including the following examples, and the changes andmodifications are also within the scope of the present disclosure asdefined in the following claims.

Example 1. Synthesis of Compound Represented by [Chemical Formula 11]

Dimethyl piperazine (87.57 mmol, 10.00 g) and hydroxyethyl chloride(87.57 mmol, 7.05 g) were mixed at a molar ratio of 1:1 in CH₃CN (40 mL)as a solvent and reaction was carried out at 100° C. for 24 hours. Afterthe reaction, the remaining hydroxyethyl chloride was removed throughextraction with diethyl ether, and then the solvent was removed by usinga vacuum pump to obtain a compound of [Chemical Formula 11] (yield85.20%) (FIG. 1).

Example 2. Synthesis of Compound Represented by [Chemical Formula 4]

N,N,N′,N′-tetraethylethylenediamine (58.0 mmol, 10.0 g) and2-bromoethanol (46.4 mmol. 5.8 g) were used in the same manner asExample 1, except that reaction was carried out at room temperature for10 hours. In this manner, a compound of [Chemical Formula 4] wasobtained (yield 80.3%) (ratio 1:0.8).

¹H NMR (300 MHz, DMSO) δ=1.15 (t, 6H), 1.25 (t, 6H), 2.80 (t, 2H), 3.01(m, 4H), 3.28-3.34 (m, 6H), 3.43 (t, 2H), 3.97 (t, 2H).

Example 3. Synthesis of Compound Represented by [Chemical Formula 5]

N,N′-dibutylpiperazine (50.4 mmol, 10 g) and 4-iodobutanol (40.3 mmol,8.1 g) were used in the same manner as Example 1, except that reactionwas carried out at room temperature for 10 hours. In this manner, acompound represented by [Chemical Formula 5] was obtained (yield 85.6%).

¹H NMR (300 MHz, DMSO) δ=0.89-0.91 (m, 6H), 1.29-1.52 (m, 8H), 1.71 (m,4H), 2.80 (t, 6H), 3.01 (t, 2H), 3.22 (t, 4H), 3.34 (t, 4H), 3.46 (t,2H).

Example 4. Synthesis of Compound Represented by [Chemical Formula 6]

1,4-diazabycyclo[2.2.2]octane (89 mmol, 10 g) and 2-bromoethanol (71.2mmol, 8.9 g) were allowed to react at room temperature for 24 hours inthe same manner as Example 1. After the reaction, the remaining1,4-diazabycyclo[2.2.2]octane and 2-bromoethanol were removed throughextraction with diethyl ether. Then, sodium trifluoroacetate (64 mmol,8.7 g) was added and the resultant mixture was agitated at roomtemperature for 12 hours. After the reaction, the resultant salt wasremoved by filtration and the solvent was removed by using a vacuum pumpto obtain a compound of [Chemical Formula 6] (yield 70%).

¹H NMR (300 MHz, DMSO) δ=2.80 (t, 6H), 3.34-3.43 (m, 8H), 3.97 (t, 2H).

Example 5. Synthesis of Compound Represented by [Chemical Formula 7]

N,N-dimethyl-N-hydroxyethylpiperazium chloride (50 mmol, 9.7 g) obtainedfrom Example 1 was allowed to react with CF₃SO₃Na (50 mmol, 8.6 g) inthe presence of CH₃CN as a solvent at room temperature for 12 hours.After the reaction, the formed NaCl was removed by filtration and thesolvent was removed by using a vacuum pump to obtain a compound of[Chemical Formula 7] (yield 95%).

¹H NMR (300 MHz, DMSO) δ=2.19 (s, 3H), 2.77-2.82 (t, 4H), 3.28-3.36 (m,7H), 3.41-3.45 (t, 2H), 3.95-4.00 (t, 2H).

Example 6. Synthesis of Compound Represented by [Chemical Formula 8]

N,N-dimethyl-N-hydroxyethylpiperazium chloride (50 mmol, 9.7 g) obtainedfrom Example 1 was allowed to react with HCO₂Na (50 mmol, 3.4 g) in thepresence of CH₃CN as a solvent at room temperature for 12 hours. Afterthe reaction, the formed NaCl was removed by filtration and the solventwas removed by using a vacuum pump to obtain a compound of [ChemicalFormula 8] (yield 96%).

¹H NMR (300 MHz, DMSO) δ=2.18 (s, 3H), 2.78-2.82 (t, 4H), 3.28-3.37 (m,7H), 3.42-3.46 (t, 2H), 3.95-3.99 (t, 2H).

Example 7. Synthesis of Compound Represented by [Chemical Formula 9]

N,N-dimethyl-N-hydroxyethylpiperazium chloride (50 mmol, 9.7 g) obtainedfrom Example 1 was allowed to react with CH₃CO₂Na (50 mmol, 4.1 g) inthe presence of CH₃CN as a solvent at room temperature for 12 hours.After the reaction, the formed NaCl was removed by filtration and thesolvent was removed by using a vacuum pump to obtain a compound of[Chemical Formula 9] (yield 96%).

¹H NMR (300 MHz, DMSO) δ=2.10 (s, 3H), 2.18 (s, 3H), 2.78-2.82 (t, 4H),3.28-3.36 (m, 7H), 3.41-3.45 (t, 2H), 3.95-3.99 (t, 2H).

Example 8. Synthesis of Compound Represented by [Chemical Formula 10]

N,N-dimethyl-N-hydroxyethylpiperazium chloride (50 mmol, 9.7 g) obtainedfrom Example 1 was allowed to react with CF₃CO₂Na (50 mmol, 6.6 g) inthe presence of CH₃CN as a solvent at room temperature for 12 hours.After the reaction, the formed NaCl was removed by filtration and thesolvent was removed by using a vacuum pump to obtain a compound of[Chemical Formula 10] (yield 97%).

¹H NMR (300 MHz, DMSO) δ=2.18 (s, 3H), 2.80 (s, 4H), 3.30-3.34 (m, 7H),3.43 (t, 2H), 3.97 (t, 2H), 4.24 (s, H).

TEST EXAMPLES Test Example 1. Determination of SO₂ Absorption

In the absorption reactor R1 as shown in FIG. 7, 0.5 g of each of themolten salts according to Examples 1-8 was dissolved into 4.5 g of waterand the following SO₂ absorption test was carried out, while thetemperature of the thermostat was maintained at 30° C. After the storagecylinder 2 was filled with SO₂ under 3 atm, an increase in weight wasmeasured as a function of time while allowing SO₂ to flow at a 30 cc/minby using a mass flow controller. After 60 minutes, the weight was notincreased any more. At that time, the amount of SO₂ dissolved in theionic liquid was calculated. Herein, NMR data obtained after thereaction of the ionic liquid obtained from the molten salt according toExample 1 with SO₂ are shown in FIG. 2.

TABLE 1 SO₂ absorption (gmole- SO₂/ gmole- Structure absorbent) Example1 [Chemical Formula 11]

1.15 Example 2 [Chemical Formula 4]

1.08 Example 3 [Chemical Formula 5]

1.14 Example 4 [Chemical Formula 6]

1.05 Example 5 [Chemical Formula 7]

0.99 Example 6 [Chemical Formula 8]

1.03 Example 7 [Chemical Formula 9]

1.20 Example 8 [Chemical Formula 10]

1.11

As shown in Table 1, the ionic liquid obtained from the molten saltaccording to Examples 1-8 shows improved SO₂ absorption as compared tothe conventional absorbent.

Test Example 2. Determination of SO₂ Absorption Depending on AbsorptionTemperature

The SO₂ absorption test was carried out in the same manner as TestExample 1 by using the ionic liquid obtained from the molten saltaccording to Example 1, setting the pressure at 1 atm and varying theabsorption temperature to 20, 40, 50 and 60° C. The results are shown inthe following Table 2.

TABLE 2 SO₂ Absorption Absorption temperature (° C.)(gmole-SO₂/gmole-absorbent) 20 2.1 40 1.15 50 1.13 60 0.85

As shown in Table 2, the ionic liquid obtained from the molten saltaccording to Example 1 shows a gradual decrease in SO₂ absorption, asthe absorption temperature is increased.

Test Example 3. SO₂ Absorption and Desorption

The SO₂ absorption was determined in the same manner as Test Example 1by using the ionic liquid obtained from the molten salt according toExample 1, and setting the pressure at 1 atm and the absorptiontemperature to 30° C. Then, the reactor was purged with nitrogen at 10mL/min to desorb SO₂. After completing the first cycle of SO₂ absorptionand desorption, absorption/desorption was repeated four times under thesame condition. The absorption/desorption tendency is shown in FIG. 3.Herein, after carrying out absorption/desorption five times, NMR dataare shown in FIG. 4.

As shown in FIG. 3 and FIG. 4, the ionic liquid according to the presentdisclosure maintains absorbability/desorbability similar to that of thefirst cycle even after being used repeatedly (FIG. 3). When comparingthe NMR data (FIG. 4) after repeating absorption/desorption five timeswith the NMR data (FIG. 1) of the molten salt according to Example 1before carrying out absorption/desorption, it can be seen that themolten salt shows no change in structure even after repeatingabsorption/desorption.

Test Example 4. Determination of SO₂ Concentration Emitted when UsingGas Containing High-Concentration SO₂

An absorption test was carried out by using gas containing 1000 ppm ofSO₂ and the ionic liquid obtained from the molten salt according toExample 1 in the SO₂ absorption-desorption system.

FIG. 5 is a graph illustrating the SO₂ concentration obtained when theionic liquid prepared from the molten salt according to Example 1 orwater is applied to a SO₂ absorption-desorption system in the presenceof gas containing SO₂ at a high concentration.

As shown in FIG. 5, when using water instead of the ionic liquidaccording to Example 1, about 700 ppm of SO₂ is emitted. On thecontrary, when using the ionic liquid according to Example 1, about 10ppm of SO₂ is emitted, suggesting a significant decrease inconcentration of discharged SO₂. Thus, it can be seen that SO₂ isremoved at a ratio of 98.8%.

Test Example 5. Determination of Solidification Depending on Presence ofHydroxyl Group

FIG. 6A shows a photographic image taken after the reaction of the ionicliquid containing a different concentration of hydroxyl group-freediamine salt with SO₂, and FIG. 6B shows a photographic image takenafter the reaction of the ionic liquid containing a differentconcentration of the molten salt according to Example 1 with SO₂.

As shown in FIG. 6A and FIG. 6B, after absorbing SO₂ by using an aqueoussolution containing 5 M of a diamine-based salt having no hydroxylgroup, the ionic liquid is solidified by absorbing SO₂ (FIG. 6A). Suchprecipitation of salt may cause a phenomenon, such as pipe blockage,during a process for absorbing and separating SO₂.

On the contrary, when using the ionic liquid containing 5 M of themolten salt having a hydroxyl group according to Example 1, the ionicliquid maintains a liquid state after absorbing SO₂ (FIG. 6B).

Test Example 6. Determination of Carbon Dioxide Absorption

An absorption test was carried out in the same manner as Test Example 1by using the ionic liquid obtained from the molten salt according toExample 1, except that CO₂ gas was introduced instead of SO₂. Afterdetermining the absorbed amount, no CO₂ absorption occurred and theionic liquid showed no change in NMR. This suggests that the ionicliquid obtained from the molten salt selectively interacts with SO₂.

Therefore, it is possible to absorb a small amount of SO₂ under theatmosphere of exhaust gas containing CO₂ at a high concentration.

[Description of Drawing Numerals] 1: Nitrogen cylinder 2: SO₂ cylinder3: Valve 4: Mass flow controller 5: Absorbent container 6: Silicone oilbath 7: NaOH solution

What is claimed is:
 1. A sulfur dioxide absorbent that is an ionicliquid which absorbs sulfur dioxide and sulfurous acid (H₂SO₃),comprising: a solvent; and a salt of a diamine compound that issubstituted with a hydroxyl group, that has a chemical formula selectedfrom the group consisting of Chemical Formula 1 to 3 below, and that isdissolved in the solvent:

where, in Chemical Formula 1 and 2, R₁-R₄ are the same or different andeach is independently selected from the group consisting of H, a C1-C6alkyl group, and a C1-C6 alkoxy group; and where, in Chemical Formula 1to 3, X is selected from the group consisting of Cl, Br, I, MeSO₃,CF₃SO₃, HCO₂, CF₃CO₂ and CH₃CO₂; and n is an integer of 1-10.
 2. Thesulfur dioxide absorbent according to claim 1, wherein the sulfurdioxide absorbent selectively absorbs sulfur dioxide and sulfurous acid(H₂SO₃) formed by combination of sulfur dioxide with water but not CO₂.3. The sulfur dioxide absorbent according to claim 1, wherein the sulfurdioxide absorbent has an absorption temperature that ranges from 10-80°C.
 4. The sulfur dioxide absorbent according to claim 1, wherein thesulfur dioxide absorbent has a desorption temperature that ranges from50-150° C.
 5. The sulfur dioxide absorbent according to claim 1, whereinthe sulfur dioxide absorbent is selected from the group consisting ofChemical Formula 4 to 11 below:


6. The sulfur dioxide absorbent according to claim 1, wherein thechemical formula is Chemical Formula
 2. 7. The sulfur dioxide absorbentaccording to claim 1, wherein the chemical formula is Chemical Formula 2and X is Cl, Br, or I.
 8. A sulfur dioxide absorbent that is an ionicliquid which absorbs sulfur dioxide and sulfurous acid (H₂SO₃),comprising: a solvent; and a salt of a diamine compound that issubstituted with a hydroxyl group, that has a chemical formula selectedfrom the group consisting of Chemical Formula 1 to 3 below, and that isdissolved in the solvent:

where, in Chemical Formula 1 and 2, R₁-R₄ are the same or different andeach is independently selected from the group consisting of H, a C1-C6alkyl group, and a C1-C6 alkoxy group; and where, in Chemical Formula 1to 3, X is selected from the group consisting of Cl, Br, I, CF₃SO₃, andCF₃CO₂; and n is an integer of 1-10.
 9. The sulfur dioxide absorbentaccording to claim 8, wherein the sulfur dioxide absorbent is selectedfrom the group consisting of Chemical Formula 4 to 7, 10, and 11 below:


10. The sulfur dioxide absorbent according to claim 8, wherein thesulfur dioxide absorbent selectively absorbs sulfur dioxide andsulfurous acid (H₂SO₃) formed by combination of sulfur dioxide withwater but not CO₂.
 11. The sulfur dioxide absorbent according to claim8, wherein the sulfur dioxide absorbent has an absorption temperaturethat ranges from 10-80° C.
 12. The sulfur dioxide absorbent according toclaim 8, wherein the sulfur dioxide absorbent has a desorptiontemperature that ranges from 50-150° C.
 13. The sulfur dioxide absorbentaccording to claim 8, wherein the chemical formula is Chemical Formula2.
 14. The sulfur dioxide absorbent according to claim 8, wherein thechemical formula is Chemical Formula 2 and X is Cl, Br, or I.
 15. Asulfur dioxide absorbent that is an ionic liquid which absorbs sulfurdioxide and sulfurous acid (H₂SO₃), comprising: a solvent; and a salt ofa diamine compound that is substituted with a hydroxyl group, that hasChemical Formula 2 below, and that is dissolved in the solvent:

where R₁ and R₂ are the same or different and each is independentlyselected from the group consisting of H, a C1-C6 alkyl group, and aC1-C6 alkoxy group, X is chlorine (Cl), and n is an integer of 1-10. 16.The sulfur dioxide absorbent according to claim 15, wherein the sulfurdioxide absorbent selectively absorbs sulfur dioxide and sulfurous acid(H₂SO₃) formed by combination of sulfur dioxide with water but not CO₂.17. The sulfur dioxide absorbent according to claim 15, wherein thesulfur dioxide absorbent has an absorption temperature that ranges from10-80° C.
 18. The sulfur dioxide absorbent according to claim 15,wherein the sulfur dioxide absorbent has a desorption temperature thatranges from 50-150° C.